* consistent efficient performance
* substantially longer service life with minimal disposal requirements
* universal availability
* independent of toxic and limited resources
* small vehicle to generating station capacity
* well developed technology
* low weight and low capital cost
The Refrigerated Compression Gas Turbine (RCGT) described by a Cryogenic Brayton Cycle offers least refrigerant consumption of the cryogenic engines, which are at various stages of development. The gas turbine has several advantages including ability to burn most fuels or run on recovered heat. Reliability is high while maintenance, weight and emissions are low. The gas turbine is potentially a universal prime mover, however it is inefficient in small engines, especially in variable speed vehicle application. Low compression work of the RCGT extends high efficiency to small units and the gas turbine is readily converted to injection of liquid air or nitrogen into an external and independently driven compressor.
In automotive application RCGT operating efficiency is 4 times higher than for a normally aspirated engine. In stationary application operating efficiency is 2.5 times higher than for a micro-turbine. Implications of RCGT fuel consumption on emissions and fuel selection with respect to storage, safety, and cost are profound; especially advantageous to Hydrogen, which might be cogenerated using waste heat of air liquefaction. In addition, the technology has the unique advantage of competitive cost at inception, primarily due to the price of liquid air at less than 3 US c/lb [2] and convertibility of turbo-chargers to turbines. A hydrogen fueled RCGT is more efficient than a fuel cell and can be implemented years ahead. Operating efficiency obviously does not take into account the renewable energy required for air liquefaction, nor does it account for fuel refining, distribution and exploration. In addition, a complete evaluation of the RGTC and the Liquid Air Economy must include the impact of reduced fuel consumption on alternate fuel development and emissions, handling safety associated with both fuel and liquid air, and alternate energy storage.
Production of liquid air or nitrogen requires about 40% of RCGT power based on a liquefier Figure of Merit of 0.5 [3], a number already in practice. This figure does not include the benefit of cogeneration of fuel and space heating using liquefier waste heat. The refrigerant will be available with uniform distribution from a combination of wind and hydro [4] sources, supplemented by off-peak grid, vehicle recovery and geothermal.
It is useful to re-define some terms associated with renewable energy before proceeding with a discussion involving the dual fluid (fuel and oxidizer/coolant) liquid air engine. The terms "tank-to-wheel” and “well-to-wheel” [5] used to describe vehicle efficiency including refining, distribution and exploration of fuel, are used herein as “operating” and “total”, respectively, for inclusion of stationary engines, non-fuel heat addition and liquefaction of air.
Background
It is useful to re-define some terms associated with renewable energy before proceeding with a discussion involving the dual fluid (fuel and oxidizer/coolant) liquid air engine. The terms "tank-to-wheel” and “well-to-wheel” [5] used to describe vehicle efficiency including refining, distribution and exploration of fuel, are used herein as “operating” and “total”, respectively, for inclusion of stationary engines, non-fuel heat addition and liquefaction of air.
Background
A Liquid Nitrogen Economy or liquid air economy was proposed in the early 1970's [6] by Kleppe and Schneider shortly after a cryogenic engine [7] was patented by Boese and Hencey. Both the engine and the proposed Economy were limited to ambient engine inlet gas temperature. Subsequently some Rankine cycle expansion engines with sub-ambient compression have been built and tested. These include a fired stationary gas turbine [8] by Mitsubishi Corp. and a fuel-less liquid nitrogen engine with an ambient heated quasi-isothermal expander [9] utilizing a frost free-heat exchanger [10] for sub-compact vehicles by the University of Washington. More advanced concepts have been proposed to reduce refrigerant consumption, including a Brayton cycle for fuel-less operation with ambient heating and sub-ambient cooling [11] by the University of North Texas. The author of this blog has proposed a similar Brayton cycle with addition of over-ambient heating [12, 13]. Highview Power Storage is presently going forward with deployment of cryogenic storage technology based on the fuel-less engine [14] of Peter Dearman, which was proto-typed in the early 2000's.
Operation
Two modes of operation are considered for both a stationary and an automotive gas turbine;
* Power, in which the gas turbine operates in a modified Brayton cycle with quasi-isothermal compression due to liquid air injection.
* Storage, in which renewable energy drives a liquefier to produce liquid air.
Power Mode
Refrigerated compression increases the source to sink temperature difference in a modified Brayton Cycle by injection of refrigerant upstream of the compressor. Stored energy of refrigerant is added by lowering the sink temperature just as stored energy of fuel is added by raising the source temperature. Regeneration increases cycle efficiency and is included between the sink and ambient to cool intake air just as for heating intake air to the source. Quasi-isentropic compression increases working fluid density while lowering compression work. The preferred refrigerant is liquid air or nitrogen because it is always available. Carnot operating efficiency is over 90% with typical turbine inlet gas temperature of 1600 F and liquid air temperature of -300 F. Even in an unheated system Carnot operating efficiency exceeds 70%. High Carnot operating efficiency translates to high efficiency of the actual cycle, increasing inversely with respect to compression ratio [6], except as limited by recuperator or regenerator effectiveness. In low capacity engines, operating efficiency of the RBC is about 2.5 times as compared to a micro-turbine and about 4 times as for a gasoline engine. Liquid air replaces between 60% and 80% of the fuel in vehicle application and between 40% and 60% in stationary application, depending upon engine size and compression ratio.
Storage Mode
Renewables for stationary use include building amplified wind and station off-peak as well as natural sources. Renewables available for vehicle use include deceleration, draft, shock and solar. It is important to economize refrigerant consumption, especially in motor vehicle use, because of refrigerant weight limitation. Low fuel consumption is advantageous to development of alternative fuels, and cogeneration of fuel with waste heat of refrigerant liquefaction provides further advantage. Development of an efficient refrigerant machine for vehicle use will increase refrigerant mileage.
Comparison of RCGT performance with other engines needs to include both liquid air and fuel preparation energy to determine total efficiency. Preparation of fuel includes refining, distribution and exploration, however preparation of liquid air involves only refining (liquefaction) , since it does not have to be distributed or explored for. Refining of fuel such as coal or gasoline presently requires intense energy use, generally not amenable to renewable energy input such as wind and solar. Preparation of liquid air may utilize several advanced processes, including; compression/wet expansion [15], magnetic and thermo-acoustic, which are amenable to wind, solar and other renewable input. Compression/Wet Expansion is selected as the Reference Liquefier in the Liquid Air Economy because it is expected to perform as well as the other more exotic types, while requiring less development work.
Performance and Cost
Table 1 presents operating efficiency, total efficiency, mass ratio of liquid air to gasoline (Lqa/G), and gasoline + liquid air cost for a range of RCGT pressure ratio (Pr), applicable to vehicle capacity from 7 to 28 kW at 50 mph. An Otto cycle vehicle engine is included for comparison.
Table 1
Pr Op. eff. Ttl. eff. Lqa/G Cost
(%) (%) (c/mi)
RCGT 1.50 51 19 43 12
RCGT 1.50 51 19 43 12
2.00 64 27 33 8
3.00 70 30 31 7
Otto 10.00 18 14 -- 11
Refrigerant consumption is further improved by addition of hybrid drive with regenerative braking. Assumed costs are $4.00 US/gal of gasoline and $0.25 US/gal of liquid air. RCGT fuel cost is further reduced by use of a lower grade and refrigerant cost by utilizing liquefier waste heat in cogeneration of renewable fuel.
Table 2 presents operating efficiency and total efficiency for a solar heated RCGT with a pressure ratio of 3, applicable to distributed generation capacity of 28 kW. Efficiency of a turbine with ambient air compression and battery storage is included for comparison.
Table 2 presents operating efficiency and total efficiency for a solar heated RCGT with a pressure ratio of 3, applicable to distributed generation capacity of 28 kW. Efficiency of a turbine with ambient air compression and battery storage is included for comparison.
Table 2
Op. eff. Ttl. eff.
(%) (%)
RCGT 70 30
Ambient Cmp. turbine later
Refrigerant economy is improved by utilizing liquefier waste heat in cogeneration, including renewable fuel in station application and space heating in local application.
References
1. HighView Power Storage, Media Archive, 2012
www.highview-power.com/wordpress/?page_id=2189
2. Fan, K., "Price of Liquid Nitrogen", The Physics Factbook, 2007
3. Guy, K. FREng, FCGI, "www.gawdawiki.org/wiki/LIN_Production_Economics", Espirit Associates, 2011
4. Acker, T., "Integration of Wind and Hydropower Systems", International Energy Agency, Task 24, 2011
5. Brinkman, N. et-al, “Well-to-wheels Analysis of Advanced Fuel/Vehicle Systems”, General Motors and Argonne National Laboratory, 2005
6. Kleppe, J. and Schneider, R., "A Nitrogen Economy", ASEE, 1974
7. Boese, H. and Hencey, T., "Non-Pollution Motors Including Cryogenic Fluid as the Motive Means", U.S. Patent 3,681,609, 1972
8. Kishimoto, K. et-al, “Development of Generator of Liquid Air Storage Energy System”, Mitsubishi Heavy Industries Technical Review Vol. 35 No. 3, 1998
7. Boese, H. and Hencey, T., "Non-Pollution Motors Including Cryogenic Fluid as the Motive Means", U.S. Patent 3,681,609, 1972
8. Kishimoto, K. et-al, “Development of Generator of Liquid Air Storage Energy System”, Mitsubishi Heavy Industries Technical Review Vol. 35 No. 3, 1998
9. Knowlen, C. et al, "High
Efficiency Energy Conversion Systems for Liquid Nitrogen Automobiles", University of Washington, SAE
981898, 1998
10. Knowlen, C. et al,"Fost-Free Cryogenic Heat Exchangers for Automotive Propulsion", AIAA 97-3168,Joint Propulsion Conference, 1997
11. Ordonez, C.,
“Liquid Nitrogen Fueled, Closed Brayton Cycle Cryogenic Heat Engine”, Energy Conversion
and Management 41, 200012. Kaufman, J. "Vehicle Power Assist by Brake, Shock, Solar and Wind Energy Recovery", U.S. Patent 7,398,841 B2, 2008
13. Kaufman, J. "Motor Vehicle Energy Converter", U.S. Patent 7,854,278B2, 2010
14. Dearman, P. and Highview Entpr. Ltd., "European Patent Office Patent No. KR20080007234 (A), 2008
15. Bond, T., "Replacement of Joule-Thompson Valves by Two Phase Flow Turbines in Industrial Refrigeration Applications", IMECHE Conf. Transactions, Vol. 6, 1999
